fswatch to watch only a certain file extension

I am using fswatch and only want it triggered if a file with extension .xxx is modified/created etc. The documentation and the second reference below indicate that:

• All paths are accepted by default, unless an exclusion filter says otherwise.
• Inclusion filters may override any exclusion filter.
• The order in the definition of filters in the command line has no effect.

Question: What is the regular expression to use to exclude all files that do not match the .xxx extension?

References:

• Is there a command like "watch" or "inotifywait" on the Mac?
• Watch for a specific filetype

Platform:

• MacOS 10.9.5.

I'm fswatch author. It may not be very intuitive, but fswatch includes everything unless an exclusion filter says otherwise. Coming to your problem: you want to include all files with a given extension. Rephrasing in term of exclusion and inclusion filters:

• You want to exclude everything.
• You want to include files with a given extension ext.

That is:

• To exclude everything you can add an exclusion filter matching any string: .*.

• To include files with a given extension ext, you add an inclusion filter matching any path ending with .ext: \\.ext$. In this case you need to escape the dot . to match the literal dot, then the extension ext and then matching the end of the path with $.

The final command is:

$ fswatch [options] -e ".*" -i "\\.ext$"

If you want case insensitive filters (e.g. to match eXt, Ext, etc.), just add the -I option.